Method of producing a dielectric lens for an antenna and dielectric lens obtainable by said method

Abstract: A dielectric lens which is produced by injection molding of an expandable material consisting of a synthetic resin, a foaming agent and a dielectric constant conditioner. The expandable material is injected into a cavity of a foaming mold up to at least about 80 percent by weight and at least about 100 percent by volume of the capacity of the cavity and is foamed at an expansion ratio of not more than about 1.3. During the foam molding, the surface of the foam-molded body is solidified to be a radome layer. Then, the foam-molded body is transferred from the foaming mold to a shaping mold, in which the foam-molded is cooled down naturally. Further, a fitting tab is provided integrally with the foam-molded body at the edge of the radome layer.

Description: The present invention relates to a manufacturing process for a dielectric lens used as an element of an antenna for receiving microwave for communication and broadcasting. A conventional dielectric lens used as an element of an antenna for receiving microwave of 5GHz or more is conventionally made of a resin, for example, polypropylene, polyethylene, polystyrene or the like. Ceramic powder, which acts as a foaming agent and as a dielectric constant conditioner is added, and the resin is foamed and molded into a dome. Such a conventional dielectric lens is generally produced by injection molding. However, in producing a thick product by ordinary injection molding, there occur a sink mark on the surface and a lot of voids inside. Therefore, injection compression molding and structural foaming are recently suggested. Even a thick product by the injection compression molding does not have defects such as a sink mark and a void, and additionally the product can obtain a substantially fixed dielectric constant entirely. However, the injection compression molding requires a mold of a complicated structure and an exclusive molding machine, and thus, the facilities are costly. The structural foaming solves the problem about a sink mark and a void. However, a product by the structural foaming varies in the expansion ratio and in the dielectric constant from portion to portion, and further, a swirl mark on the surface is caused by bubbles. In the foaming and molding of the conventional dielectric lens, the surface is solidified to be a radome layer. The radome layer protects the inner foamy body from weathering and reinforces the foamy body. However, if the molded lens is taken out of the mold before the radome layer is formed sufficiently thick, the radome layer will be deflected by the expanding force of the foamy body. On the other hand, if the mold is cooled too suddenly or if the mold cooling time is too long, the radome layer will be formed too thick, which lowers the characteristics as a lens. Further, the long mold cooling time lengthens a molding cycle and lowers the production efficiency. Further, in order to fabricate the dielectric lens as an element of an antenna, the dielectric lens must be provided with a fitting tab which is to engage with a bracket. Conventionally, insert molding and sandwich molding are taken for providing the fitting tab. The insert molding is carried out as follows: a fitting tab, which is made of a high strength resin or a metal, is inserted into a mold; an expandable material is injected into the mold; and thus, on completion of the molding, the fitting tab is fixed on the molded article (dielectric lens). In this method, however, a step of making the fitting tab and a step of inserting the fitting tab into the mold are necessary, which requires more cost and time. In the sandwich molding, a radome layer and a foamy body are made of different resins. The sandwich molding is carried out as follows: a radome layer and a fitting tab are integrally made of a high strength resin by injection molding; and an expandable material is injected into the molded article (radome layer) and becomes a foamy body therein. This method, however, requires two injection cylinders and two kinds of materials. US-A-4,482,513 relates to a method of molding foam/aluminum flake microwave lenses. A microwave lens is manufactured by selecting a mold having a cavity of appropriate configuration, preheating the mold, and filling the mold with a mixture of low density polyurethane foam component and fine aluminum flakes evenly dispersed throughout the foam structure allowing the mixture to cool and form a body of the appropriate configuration. US-A-5,154,973 relates to a composite material for dielectric lens antennas having a high dielectric constant ceramic and a micromolecular material. The dielectric lens antenna is formed by an injection molding machine. It is the object of the present invention to provide a method of producing a dielectric lens for an antenna which has no sink marks and no swirl marks on the surface, and no voids inside and which electrical characteristics, such as dielectric constant and Q, are fixed entirely, wherein the method requires a mold of comparatively simple structure and of low cost, wherein a radom layer with a desired thickness can be formed without deflection and the molding cycle is short. This object is achieved by a method as defined in claim 1. Any synthetic resin can be used, in the inventive method as long as it can bring out a dielectric constant sufficiently high as a dielectric lens and is proper for injection foam molding. For example, polypropylene, polyethylene, polystyrene, polybutylene terephthalate, ABS resin and the like can be used. It is also possible to use a mixture of such a synthetic resin and dielectric ceramics, glass fiber or the like. As the foaming agent, a conventional agent, such as carbon dioxide azo-dicalvonamide, p, p-oxibenzenesulfonic hydrazide, or the like can be used. Because of the foaming agent, the material injected in the mold has a force against the pressure applied from outside, and accordingly, superficial defects (sink marks and swirl marks) and internal defects (voids) of the molded body can be prevented. The mixing ratio of the foaming agent depends on the desired density of the dielectric lens. However, generally, the foaming agent is added at a ratio within a range from about 0.05 to 3.0 percent by weight of the synthetic resin. If the mixing ratio of the foaming agent is less than about 0.05 percent by weight, the effect of preventing defects will not be sufficiently brought out. If the mixing ratio of the foaming agent is more than about 3.0 percent by weight, although a pressure is applied from outside, the expansion ratio will be over 1.3, and in this case, the molded dielectric lens will be poor in the inductivity and other electric characteristics. A pressure is applied from outside during the foam molding. The expansion of the material by the foaming agent contained therein is inhibited by the pressure, and thereby, a dense body can be made. In the method, the expandable material is injected up to at least about 80 percent by weight and at least about 100 percent by volume of the capacity of the cavity. Preferably, the expandable material is injected up to a percent within a range from about 85 to 91 percent by weight of the capacity of the cavity. If the expandable material is injected over 91 percent by weight of the capacity, a burr occurs, and a defective will be made. If the expandable material is injected up to a percent less than about 85 percent by weight of the capacity, the molded body will be too low in the dielectric constant to have a sufficient antenna gain. The expandable material is foamed at an expansion ratio of not more than about 1.3. Preferably, the expansion ratio is within a range from about 1.00 to 1.17. If the expansion ratio is over 1.17, the molded body is likely to be too low in the dielectric constant to have a sufficient antenna gain. If the expansion ratio is less than 1.0, the molded body is likely to have superficial defects and internal defects. In the method, since the foam-molded body is taken out of the foaming mold while the solidification of the radome layer is still light, the foam-molding cycle takes only a short time, and the foaming mold can be used efficiently. The foam-molded body still continues foaming in the shaping mold. However, since the foam-molded body is provided with a proper pressure inside the cavity of the shaping mold, the foam-molded body is not deflected. Also, the radome layer does not grow in the shaping mold any more, and the radome layer is completely solidified to be about 10mm or less in thickness, which will never degrade the characteristics as a lens. These and other objects and features of the present invention will be apparent from the following description with reference to the accompanying drawings, in which: Fig. 1 is a sectional view of a dielectric lens which is a first embodiment of the present invention, explaining a foam-molding step; Fig. 2 is a sectional view of the dielectric lens produced through the step shown in Fig. 1; Fig. 3 is a sectional view of a dielectric lens, explaining a foam-molding step of a method which is a second embodiment of the present invention,; Fig. 4 is a sectional view of a dielectric lens, explaining a shaping step of the method of the second embodiment; Fig. 5 is a perspective view of a dielectric lens produced by the method of the second embodiment; and Fig. 6 is a sectional view of the dielectric lens shown in Fig. 5. Preferred embodiments of the present invention are hereinafter described with reference to the accompanying drawings. First Embodiment First, an expandable material was prepared by mixing a synthetic resin, glass fiber and a foaming agent. As the synthetic resin, polypropylene (FR-PP, grade K7000 manufactured by Mitsui Petrochemical Co., Ltd.) was mixed at a mixing ratio by weight of 100. The glass fiber was added at a mixing ratio by weight of 10, and as the foaming agent, azo-dicalvonamide (Polyvan 206 manufactured by Mitsubishi Petrochemical Co., Ltd.) was added at a mixing ratio by weight of 0.5. For including the foaming agent with the synthetic resin, any proper method, for example, a master-batch method, a compound method or the like, can be adopted. The expandable material was injected into a cavity between an upper segment 31 and a lower segment 32 of a mold in the following condition. temperature of upper segment: 20°C temperature of lower segment: 60°C pressure of injection: 1448kg/cm 2 speed of injection: 114cm 3 /sec Then, a pressure was applied to the mold in the following condition, and the expandable material was foamed. pressure applied: 434.4kg/cm 2 pressure applying time: 20 seconds cooling time after application of pressure: 540 seconds The expandable material was injected into the cavity up to 87.2 percent by weight of the capacity of the cavity. That is, the weight of the injected expandable material was 87.2% of the theoretical limit weight which is figured out by multiplying the volume of the cavity by the specific gravity of the expandable material. The volume of the injected expandable material was equal to or more than the volume of the cavity. More specifically, the volume of the cavity was 2702.4cm 3 , and the volume of the injected material was 2763.2cm 3 . Fig. 2 shows a dielectric lens 33 produced by the above-described method. The dielectric lens 33 had no sink marks and no swirl marks on the surface and no voids inside. The dielectric constants of various portions of the dielectric lens 33 were measured, and as a result, it was confirmed that the dielectric lens 33 had a substantially fixed dielectric constant in every portion. The following Table shows the antenna gains (dB) of dielectric lenses which were produced in the above-described condition at various expansion ratios. Judging from the antenna gain, sample numbers 1 through 4 are inferior. Therefore, in the above method, the expansion ratio should be set about 1.17 or less. Second Embodiment A method of the second embodiment has a foaming step shown by Fig. 3 and a shaping step shown by Fig. 4. An expandable material was prepared by mixing a resin with a foaming agent. As the resin, polypropylene was mixed at a mixing ratio by weight of 98, and as the foaming agent, azo-dicalvonamide was mixed at a mixing ratio by weight of 2. Further, CaTiO 3 which acts as a dielectric constant conditioner was added. The expandable material was injected from a cylinder into a cavity 13 of a foaming mold 10. As the dielectric constant conditioner, BaTiO 3 , MgTiO 3 or the like can be used as well as CaTiO 3 . The foaming mold 10 consists of a fixed upper segment 11a and a movable lower segment 11b. These segments 11a and 11b are made of a metal with a high coefficient of thermal conductivity, such as copper, iron or the like, and has temperature regulation holes 12 through which a coolant circulates. The cavity 13 is a dome with a radius of 90mm, and a foam-molded body 1 thereby will be the shape. The injection foam molding was carried out under the following condition. temperature of cylinder: 220°C temperature of mold: 80°C pressure of injection: 1448kg/cm 2 speed of injection: 114cm 3 /sec pressure applied: 434.4kg/cm 2 cooling time: 180 seconds After the injection of the expandable material, the mold was kept at a temperature of 80°C for the cooling time. During the cooling time, the injected material was foamed and was lightly solidified on the surface, and thus, a radome layer 3 was formed on the surface of a foamy body 2. After the cooling time, the foam-molded body 1 was taken out of the foaming mold 10 and transferred to the shaping step. A shaping mold 20 consists of a main segment 21 with a cavity 22, and a movable plate 25 which is movable up and down along guide poles 23. The movable plate 25 is provided with a specified pressure by a cylinder 26 and presses the foam-molded body 1 placed in the cavity 22. The cavity 22 is identical with the foam-molded body 1. The main segment 21 and the movable plate 25 are made of a material with a low coefficient of thermal conductivity, such as a compact of wooden flour with ABS resin or ceramics. ABS resin has a coefficient of thermal conductivity of 5 x 10 -4 cal/cm·S·°C. Alumina, which is a typical kind of ceramics, has a coefficient of thermal conductivity of 4 x 10 -3 cal/cm·S·°C. Also, the main segment 21 and the movable plate 25 can be made of a metal, but in that case, a temperature regulating system is necessary. The foam-molded body 1 taken out of the foaming mold 10 was placed in the shaping mold 20 immediately. In the shaping mold 20, a pressure of 5.75kg/cm 2 was applied to the foam-molded body 1 by the movable plate 25, and the foam-molded body 1 was kept under the pressure for one hour. In the meantime, the foam-molded body 1 was cooled down naturally. Although the foamy body 2 still continued foaming, the radome layer 3 was regulated by the shaping mold 20 and was solidified without deflection. The solidification of the radome layer 3 completed in the shaping mold 20. Because the foam-molded body 1 was naturally cooled down in the shaping mold 20 and because the material of the shaping mold 20 has a low coefficient of thermal conductivity (lower than the coefficient of thermal conductivity of the material of the foaming mold 10), the radome layer 3 did not become thick. A dielectric lens produced in this way had an expansion ratio of 1.15, and a dielectric constant of 2.1. The thickness of the radome layer 3 was 5mm, and the accuracy in the shaping as a dome was +/-0.5mm or less. The dielectric lens had no sink marks, no swirl marks and no voids. Now referring to Figs. 5 and 6, a modification of the second embodiment is described. A dielectric lens 1 was produced basically in the above-described method of the second embodiment. The dielectric lens 1 has a foamy body 2 inside, a radome layer 3 on the surface and further fitting tabs 4 on the circumference. The fitting tabs 4 were provided to the dielectric lens 1 by integral molding. In this case, the cavity of the mold has recesses for forming the fitting tabs 4. The expandable material flew into the recesses, and the material in these recesses was solidified to be fitting tabs 4 simultaneously with the solidification of the radome layer 3. After the molding and the cooling, holes 5 were made in the fitting tabs 4 by drilling. In such a case of providing fitting tabs 4, the shaping step can be eliminated. In a method without the shaping step, as long as the same expandable material and the same type of foaming mold as in the second embodiment are used, the following molding condition is proper. temperature of cylinder: 230°C temperature of mold: 60°C pressure of injection: 1448kg/cm 2 speed of injection: 114cm 3 /second pressure applied: 434.4kg/cm 2 The mold is made of a metal with a high coefficient of thermal conductivity, such as copper, iron or the like, and the mold is kept at a temperature of 60°C by a coolant circulating therein. After injection of the expandable material, when a time proper for obtaining a desired state of foaming of the foamy body 2 and of solidification of the radome layer 3, for example, 90 seconds has passed, the molded body (dielectric lens 1) are taken out of the mold. Then, the molded body is cooled down in the air. The growth of the radome 3 depends on the temperature of the cavity of the mold and the cooling time after injection. Under the above condition, the radome 3 grows to be 5mm in thickness. In the point of the characteristics as a lens, the thickness of the radome 3 is preferably 10mm or less. In order to obtain a desirably grown and sufficiently firm radome 3 and sufficiently firm fitting tabs 4, further considering shortening of time, the mold is preferably kept at a temperature within a range from about 50 to 70 °C for about 80 to 100 seconds. After the dielectric lens 1 is taken out of the mold, the foamy body 2 still continues foaming a little. However, since the radome 3 is almost solidified, the dielectric lens 1 will never be deflected by the expanding force.

Claims: A method of producing a dielectric lens for an antenna, the method comprising: a foam-molding step in which an expandable material whose main constituent is a synthetic resin is injected into a cavity (13) of a foaming mold (10) to obtain a dome body (2); characterized in that after injection of said expandable material, the mold (10) is kept at a special temperature for the cooling time for solidifying an outer surface to obtain a thin radome layer (3) on said outer surface of said dome body (2); after the cooling time, said dome body (2) is transferred to a shaping mold (20) for further solidification of the radome layer (3); and said method further comprises the step of press forming the foam-molded body (2,3) in the shaping mold (20) to integrally add to the foam-molded body (2) with the thin radome layer (3) fitting tabs (4), wherein a pressure is applied to the dome body in the step of press forming. A method of producing a dielectric lens for an antenna as claimed in claim 1, wherein the synthetic resin is selected from the group consisting of polypropylene, polyethylene, polystyrene, polybutylene terephthalate and ABS resin. A method of producing a dielectric lens for an antenna as claimed in claim 1, wherein the foaming agent is selected from the group consisting of carbon dioxide, azodicalvonamide and p,p-oxibenzenesulfonic hydrazide. A method of producing a dielectric lens for an antenna as claimed in claim 1, wherein the expandable material further contains a dielectric constant conditioner. A method of producing a dielectric lens for an antenna as claimed in claim 1, wherein after injection of the expandable material, same is provided with a pressure, and said expandable material is injected up to at least about 80 percent by weight and at least about 100 percent by volume of a capacity of the cavity (31,21) and is foamed at an expansion ratio of not more than about 1.3. A method of producing a dielectric lens (33) for an antenna as claimed in claim 5, wherein the expandable material is injected up to a percent by weight of the capacity of the cavity (31,32) within a range from about 85 to about 91. A method of producing a dielectric lens (33) for an antenna as claimed in claim 5, wherein the expandable material is foamed at an expansion ratio within a range from about 1.00 to about 1.17. A method of producing a dielectric lens (33) for an antenna as claimed in claim 5, wherein the synthetic resin is selected from the group consisting of polypropylene, polyethylene, polystyrene, polybutylene terephthalate and ABS resin. A method of producing a dielectric lens (33) for an antenna as claimed in claim 5, wherein the foaming agent is selected from the group consisting of carbon dioxide, azodicalvonamide and p,p-oxibenzenesulfonic hydrazide. A method of producing a dielectric lens (33) for an antenna as claimed in claim 5, wherein the expandable material further contains a dielectric constant conditioner. A dielectric lens for an antenna obtainable by a method according to any of claims 1 to 4, comprising: a dome foamy body (29 which is injection-molded out of an expandable material whose main constituent is a synthetic resin; characterized by a radome layer (3) which is formed on a surface of the dome foamy body (2) as a solid body with a specified thickness with the molding of the dome foamy body (2); and a fitting tab (4) which is integral with the radome layer (3) and extends outward from the radome layer (3).

Inventors: NAKAMURA TATSUHIRO, NONOGAKI HIROSHI, TANINO YOSHITAKA, YAMAMOTO KEIZO, NAKAMURA, TATSUHIRO, NONOGAKI, HIROSHI, TANINO, YOSHITAKA, YAMAMOTO, KEIZO
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1994
Main classification: H01Q15
IPC codes: H01Q15